The present invention relates to a method of forming a semiconductor thin film, and more particularly to a pulse laser irradiation method for irradiating a pulse laser onto a non-single crystal semiconductor thin film as an active layer of a polysilicon thin film transistor formed over an insulative substrate to apply the same to a liquid crystal display or an image sensor.
In recent years, the manufacturing technique for manufacturing a polysilicon thin film transistor has been applied to form a liquid crystal display with a driver circuit over an inexpensive glass substrate. An excited dimer laser crystallization method is applied to form the polysilicon thin film in view of a reduction in temperature of the process and a high throughput, wherein the excited dimer laser is irradiated onto an amorphous silicon thin film to cause a crystallization of the amorphous silicon thin film, thereby to form a polysilicon thin film.
The excited dimer laser crystallization method is, however, disadvantageous in a limited time for subjecting the amorphous silicon thin film to a heat treatment since the laser beam is a pulse laser beam. The limitation to the time for the heat treatment to the amorphous silicon thin film causes a limitation to size of crystal grains of the polysilicon of the polysilicon thin film. A field effect mobility of carriers of the polysilicon thin film transistor depends on the crystal grain size of the polysilicon thin film of the polysilicon thin film transistor, for which reason the field effect mobility of carriers is limited to about 100 cm2/Vs which is applicable to the liquid crystal display but inapplicable to a high integrated circuit driven by a high frequency such as a dynamic random access memory device.
A first conventional technique to increase the crystal grain size of the polysilicon thin film is disclosed in Japanese laid-open patent publication No. 10-275781 and also disclosed in Japan Applied Physics, 42, p. 694. In accordance with the first conventional technique, plural pulse laser beams are synthesized so that a synthesized laser beam is irradiated onto the amorphous silicon thin film to form the polysilicon thin film.
A second conventional technique to increase the crystal grain size of the polysilicon thin film is disclosed in MRS Bulletin, vol. 21, 1996 March, p. 39. A fine-line laser beam with a beam width of 5 micrometers is scan-irradiated at a pitch of 0.75 micrometers onto island-shaped amorphous silicon film films to form a uniformly grown polysilicon thin film with a almost parallel order of the crystal grain boundaries.
The above first conventional technique is, however, disadvantageous in a difficulty in realizing a uniform increase in crystal grain size over a large area such as a few hundreds millimeters squares for application to the liquid crystal display device.
The above second conventional technique is, however, disadvantageous in a drop of throughput and in requiring a complicated carrying system to realize a sub-micron stage positioning accuracy.
In the above circumstances, it had been required to develop a novel pulse laser anneal process for forming a non-single crystal semiconductor thin film free from the above problem.
Accordingly, it is an object of the present invention to provide a novel pulse laser anneal process for forming a non-single crystal semiconductor thin film free from the above problems.
It is a further object of the present invention to provide a novel pulse laser anneal process for forming a non-single crystal semiconductor thin film, which allows an increased throughput.
It is a still further object of the present invention to provide a novel pulse laser anneal process for forming a non-single crystal semiconductor thin film having a uniformly large crystal grain size over a large area.
It is yet a further object of the present invention to provide a novel pulse laser anneal process for forming a non-single crystal semniconductor thin film having a uniformly high carrier mobility over a large area.
The present invention provides a method of irradiation of plural pulse laser beams onto one position of a non-single crystal semiconductor, wherein the pulse laser beams are not higher in energy density than an energy density threshold value necessary for causing a micro-crystallization of the non-single crystal semiconductor.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.